1. Field of the Invention
The present invention is a pulse width decoder. It provides digital indications of pulse widths or durations and utilizes digital circuitry to this end.
2. Description of the Prior Art
The present invention is of particular utility in security systems such as those for the detection of fire and burglary. In systems of this kind decoding of an input signal pulse is frequently accomplished by a detector which performs a comparison between a preset reference and the width of the applied signal pulse. The input signal pulse is typically applied to a two-branch circuit. One branch provides a ramp which triggers a pulse generator when a predetermined voltage is achieved. The output of that pulse generator and a trailing pulse which is produced by a differentiating circuit in the other branch are applied to a coincidence circuit so that when that trailing pulse corresponds in time with the pulse output of the pulse generator an overall output results. That is, an overall output is produced whenever the input pulse is substantially equal in width to the preset reference. Adaptation of circuitry of this kind to the sampling of a plurality of input pulse widths involves extensive duplication of ramp circuitry and great complexity.
The prior art approach just described employs a ramp voltage and is premised on the attainment of a voltage threshold by that voltage. It will be noted that one threshold is provided. The present invention departs from the prior art by the provision of a variable divisor counter and the use. of a number of successive time thresholds. Locally generated pulses are employed and provision is automatically made for an unusual range of drift of those pulses.